New Systemic Inflammatory Indices as Predictors of Fulminant Myocarditis in Children

Background/Objectives: Myocarditis is a major cause of morbidity and mortality in children and can lead to long-term heart failure, dilated cardiomyopathy, the need for heart transplantation, or death. New systemic inflammatory indices that combine lymphocyte, neutrophil, and platelet counts have been recently used as strong prognostic markers of some inflammatory diseases and adverse outcomes of neoplasms. This study aimed to investigate the use of new systemic indices as early predictive markers for adverse outcomes in patients with pediatric myocarditis. Methods: This study retrospectively examined patients between the ages of >1 month and <18 years who were monitored in our clinic with a diagnosis of myocarditis between 1 January 2022 and 31 December 2024. The cases were divided into two groups: fulminant myocarditis (requiring the use of inotropes or extracorporeal membrane oxygenation due to hemodynamic disturbance) and non-fulminant myocarditis. The systemic inflammatory index values of these groups (calculated in the first 6 h) were compared, and the results were statistically analyzed. Results: The study included 122 pediatric myocarditis cases treated during the study period (80 boys; median age: 11 (IQR: 8–14) years). Twenty-six of these patients (21.3%) developed fulminant myocarditis. The median systemic immune-inflammation index (SII) value in the group with fulminant myocarditis was 1300 (IQR: 1000–1600), while this value was 500 (IQR: 350–650) for the non-fulminant group (p < 0.05). The median systemic inflammatory response index (SIRI) values were 2.9 (IQR: 2.5–3.2) in the fulminant myocarditis group and 1.5 (IQR: 1.2–1.8) in the non-fulminant group (p < 0.05). The cut-off values for fulminant myocarditis were found to be 1050 for the SII, with an AUC value of 0.76 (95% confidence interval: 0.80–0.96; p < 0.001), and 1.9 for the SIRI, with an AUC value of 0.64. Conclusions: The SII and SIRI may independently predict adverse myocarditis prognoses in children. These new biomarkers are easy to calculate using routine blood parameters.